Aqueous coating compositions are used for intercoating and base topcoating of automobiles. Heretofore, organic solvent type coatings have been exclusively used for automobiles. However, it is now strongly demanded to change from organic solvent type coatings to aqueous coatings in view of safety on coating process, reduction of environmental pollution, resources-saving and the like.
As the aqueous intercoating composition, for example, there have been known an aqueous intercoating composition comprising a carboxylic group-containing resin, an urethane bond-containing diol and resin particles (Japanese Patent Laid-Open Publication No. 3-52973), a thermosetting aqueous coating composition comprising a polyhydric carboxylic acid resin, an amino resin, a linear low molecular weight polyester diol and an benzoin (Japanese Laid-Open Patent Publication No. 4-93374) and the like. However, various problems are arisen when using them for intercoating.
That is, surface smoothness and final appearance are not sufficient. Further, "pinhole" as a defective appearance caused by bumping of water during curing a coated film is liable to be arisen in comparison with a conventional organic solvent type paint. This phenomenon is observed in case that the film thickness is 20 to 40 .mu.m and it becomes the cause of deterioration of the final appearance. Further, water resistance of the coating containing the linear low molecular weight polyester diol is not sufficient.
In general, a two coat/one bake coating process which comprises coating a metallic base paint containing a metallic pigment on a plate on which primer-coated and intercoated (referred to as topcoating), followed by wet-on-wet coating a clear paint without curing the metallic base coat, and then simultaneously curing the metallic base coating and clear coating, is used for coating automobiles. In the metallic base paint used for this process, a flake metal pigment (e.g. aluminum) is satisfactorily oriented so that an excellent appearance of the coated film can be obtained.
Particularly, an aqueous metallic base coating composition comprising a water dispersant of an amide group-containing acidic resin and hydrophilic group-carrying polyurethane resin as a main component (Japanese Laid-Open Patent Publication No. 4-25582) is known as to the above application. However, "pinhole" as a defective appearance is liable to be arisen when the coated film is cured with heating after a clear coating was coated and, therefore, this paint is not necessarily satisfactory.